Drunk on less than two pints (5)
I believe the answer is:
litre
'less than two pints' is the definition.
(I've seen this before)
'drunk on' is the wordplay.
'drunk' becomes 'lit' (I've seen this in another clue).
'on' becomes 're' ('on' can be mean 'with reference to').
'lit'+'re'='LITRE'
(Other definitions for litre that I've seen before include "Metric unit of volume" , "Metric measure of capacity" , "It's equal to about 1.75 pints" , "Fluid measure" , "One thousand cc" .)